Field of the Invention
The invention relates to an electric tool using a brushless motor, and particularly provides an electric tool capable of suppressing permeation of moisture or dust into an internal space of a motor, a bearing part, or a control circuit substrate, thereby increasing a device lifetime.
Description of Related Art
Currently, electric tools using a brushless direct current (DC) motor and a controller, such as a microcomputer, to control rotation of a motor at a high precision are already known. The brushless DC motor uses a sensor magnet to detect a rotational position of a rotor, and uses the controller to control a driving current supplied to a coil of the motor, thereby controlling the rotation at a high precision. The technique of Patent Literature 1 is known as an electric tool using such brushless DC motor. Here, a conventional electric tool (a disc grinder here) is described with reference to FIG. 14. FIG. 14 is a longitudinal cross-sectional view illustrating a conventional electric tool 101. A frame body (“casing” in a general sense) of the electric tool 101 is formed by a motor casing 102 accommodating a motor 106 as a driving source, a rear cover 104, and a gear box 103. In the rear cover 104, a power cord 128 connected externally and a power switch 151 turning on and off power of the electric tool 101 are disposed. The gear box 103 accommodates a driving transmission unit. The driving transmission unit includes bevel gears 122 and 132 performing approximately 90° conversion on a power transmission direction of a rotation shaft of the motor, and accommodates a spindle 131 of an output shaft of a grindstone 29. On a periphery of a rear side of the grindstone 29, a protection cover 126 preventing spreading of dust caused by cutting is disposed.
Regarding the disc grinder, sometimes the disc grinder is being operated while being held single-handed. Therefore, a diameter of a gripping part 102a for the operator to hold needs to be thin and easy to grip. Under such circumstance, the motor casing 102 is integrally formed in a substantially cylindrical shape to ensure its strength. The motor 106 is inserted from a front side of the motor casing 102, and a stator (a stator core 108 wound with a coil 112) is disposed on an outer circumferential side of the motor 106, and a rotor (a rotor core 107 and a cylindrical magnet 109 disposed on an outer circumferential part of the rotor 107) is disposed on an inner circumferential side of the motor 106. On the front and rear sides of the motor 106, a rotation shaft is axially supported by ball-type bearings 118 and 117. In addition, on a front side of the rotation shaft 110, a cooling fan 120 configured to generate a cooling wind is disposed, and on a rear side of the rotation shaft 110, a sensor magnet 114 in a cylindrical shape is disposed to detect a rotational position of the rotor. In the rear cover 104, a control circuit substrate 165 configured to mount a controller 171 controlling the motor and a rectifier circuit 167 and an inverter circuit substrate 144 configured to mount a three-phase alternate current (AC) inverter circuit generating a magnetic field for generating rotation to the coil 112 of the motor 106 are disposed. Six switch elements 166 are mounted on the inverter circuit substrate 144, and three Hall ICs 141 are disposed at positions opposite to the sensor magnet 114.